1. Field of the Invention
The present invention relates to the field of image processing and more particularly to an apparatus for reading and correcting a sensed color image on an individual pixel basis.
2. Discussion of the Background
Images are sensed and converted by a system into digital signals in a process commonly called the capture/digitization process. Degradations which are inherent in a capture system, in turn, limit the quality of the digitized image. If the degradations are repeatable and are known, it is possible to form correction factors which can be applied automatically to improve the quality of the digitized image. Two types of corrections that are of particular interest to the applicability of the present invention are pixel by pixel gain correction and pixel by pixel offset correction. Normally, these two operations are performed in a serial fashion. First, a "black correction" offset value appropriate for each pixel is subtracted from each captured pixel value. The result is then multiplied by the appropriate gain value for each pixel, resulting in a corrected image. Thus both functions, addition and multiplication, must be implemented. The corrected pixel values are often converted to a different quantization space using a digital memory look-up table, such as a RAM or ROM, programmed to provide, for example, a logarithmic or a gamma-correction quantization function.
A patent of interest for its teachings in this area is U.S. Pat. No. 4,745,466 entitled "Digital Color Image Processing Apparatus with Color Masking Processing Unit Addressed by a plurality of Multi-Bit Color Component Signals Using Various Combinations of the Bits of the Signals" by Yoshida et al.
In that patent, particularly with regards to the discussion with respect to FIG. 7-1, a solution to the problem of "shading" is proposed. The term "shading" as used in that patent implies an uneven optical image caused by various optical problems involving a light source, lens, and other factors. The shading correction solution uses RAM to store a signal value from each pixel during scanning of a white calibration plate. During normal image scanning, the stored values are used to address a ROM look-up table programmed to multiply the scanned pixel values by a number inversely proportional to the stored calibration values. Thus, the ROM look-up table functions as a divider circuit. Note that the shading correction circuit 130 is followed by gamma correction circuits 302, 305 and 308 in FIG. 8-1 of that patent, which are implemented as totally separate look-up tables.
Another patent of interest for its teachings is U.S. Pat. No. 4,891,692 entitled "Color Image Reading Apparatus Having Variable Exposure Control" by K. Outa.
In that patent, referencing specifically the discussion set forth in Columns 3 and 4, there is discussed the use of a standard white plate to correct for variations in the sensitivity of the image sensor. Each pixel of the CCD is corrected for its variation in sensitivity or in the light quantity which illuminates the corresponding point of the original image. In Column 4, a standard illumination signal is derived as Equation 1. Although a multiplier logic circuit could perform the multiplication function of the equation, it is stated that a high speed multiplier function could instead be realized by the use of a look-up table. Note that the outputs of the multiplier function 105 in FIG. 1 of that patent is followed by a logarithmic converter 109, which is implemented as a totally separate look-up table.
Some additional patents of interest that discuss solutions to gain and offset problems are U.S. Pat. No. 4,736,245 entitled "Calibration Method for Color Film Inspection System" by Seto et al. and U.S. Pat. No. 4,802,107, entitled "Offset Drift Correction Method In Color Film Inspection Apparatus" by Yamamoto et al.